Yes, that is my USB power meter. Yes, that is my phone charger‘s maximum charging current. (My phone can take more) Yes, that’s above the USB specification too.
So here’s the thing: USB 2.0 defines (through a few other terms) a maximum load by the device at 500 milliamps (mA). USB 3.0 allows 900 mA draw. A milliamp is 1/1000 of an amp, so 3A = 3,000mA.
So knowing that my phone charges through USB-C, which is a USB 3.0 connector, well, I think that 3,000mA is greater than 900mA, yes? Even better, that’s not the maximum.
For reference, if you’re not aware, wattage is the voltage times the amperage. 3 amps at 5 volts is 15 watts, because $3 \times 5 = 15$. Also for reference, USB runs at 5 volts.
According to my phone, it is capable of using the USB-C power delivery protocol to negotiate up to 45 watt charging!!! (9 amps, 10x normal load limit), and the adapter it comes with is capable of outputting 15 watts, which is (roughly) what you can see up there.
Now I have no clue, but that adapter, my phone, and the power meter if plugged in get rather hot at just 15 watts,this was after only a minute or two:
And we can potentially have up to three times as much current? Heat production is equal to the square of the current flow (amperage), which is why it’s vastly more efficient to use higher voltages. This is why long-distance power lines can carry up to 345,000 volts. (For reference, that’s usually later stepped down to around 69,000 V, then 13,800 V, then 220-440 V for industrial uses and 120-240 V for residential/commercial, if I have my math right.) That means that, in theory, everything will heat up nine times faster.
I mean sure, do by all means charge in 5 minutes. But goodness that’s.. actually a little scary to think about, especially given how small the charging cable is, which I don’t think can even handle that amperage just given the cable thickness.
So this leaves us with two questions: how is this being done, and how do we not melt everything in the electrical path? Both can be solved… with power delivery.
USB Power Delivery
USB, is fundamentally a data oriented connector , it’s primary focus is on data delivery and not power delivery. The thing is, there’s a standard for USB communications. So as long as you have two devices connected through USB, besides one being able to take the +5V off the power pin, they can both communicate… to exchange data, move pictures from your phone to your laptop, or, yes, negotiate sending more power.
There’s plenty of protocols for this, but based on what that power meter reports, my phone is either negotiating QC2.0, or QC3.0.
Qualcomm oversees the Quick Charge protocols, and these are (one set) of communications between a load (say, a phone), and a power delivering appliance (say, a wall adapter), to allow either higher voltages or higher amperages without being limited, to facilitate more power transfer.
This is, well, cool. If we already have a way to communicate just because we picked a widely used connector, why should we not use it to, well, communicate what power to send?
At some point, the USB Promoters Group introduced the world to USB PD, for.. Power Delivery. PD aware cables (USB-C wasn’t a thing yet) could negotiate for more power.
USB PD rev. 1 had six delivery modes, capable of supplying one of these combinations:
- 2A @ 5V (10W)
- 1.5A @ 12V (18W)
- 3A @ 12V (36W)
- 5A @ 12V (60W)
- 3A @ 20V (60W)
- 5A @ 20V (100W)
100 WATT CHARGING IS THE MAXIMUM‽‽‽
Okay calm down, it’s just a number.
PD rev. 2, part of USB 3.1, covers USB-C, and replaces the six pre-defines power profiles with four standard voltages, 5V, 9V, 15V, and 20V, and any possible output from 0.5W t0 100W is supported.
PD rev. 3 allows control of VBUS (output voltage) in 20 mV increments, to allow for constant-current or constant-voltage charging.
This also solves the question of how do we get safe 45W charging for my phone: Using PD rev. 2, the two devices will likely negotiate a 15V power delivery, at 3A, meaning the same amount of current is being pushed through as before, meaning the same amount of heat generation for three times the effective charging power.
…Electrical math is weird, okay?